Secure documents such as currency, passports, or identity cards are increasingly counterfeit around the world. This situation is a very critical issue for governments and society in general. For example criminal organizations may use fake passports or identity cards for human beings traffic. As reprographic technologies become more and more sophisticated, it becomes even more difficult to make a clear distinction between a fake document and the original. Document security has therefore a considerable impact on the economy of the countries and also on the victims of illicit traffic involving counterfeit documents.
Passports and identity cards are in general secure documents which contain a large number of protections, such as holograms, bar codes, encrypted data, specific papers or substrates, etc. Some protections are visible to the unaided eye (“overt” features), other protections are invisible (“covert” features) and their detection requires specific equipment.
In the patent application US 2007/0225402 the use of an ultraviolet luminescent ink is disclosed, which is printed in form of indicia onto the document. The ultraviolet luminescent ink is invisible under natural light, such that the indicia can be only revealed under irradiation with U.V. light. This ultraviolet luminescent ink is useful for applying codes onto security documents such as passports or banknotes. In the cited document, printing processes, including silk screen, gravure, letterpress and offset printing are used to apply the invisible ultraviolet fluorescent inks.
Luminescent compounds in pigment form have been widely used in inks and other preparations (see U.S. Pat. No. 6,565,770, WO08033059, WO08092522) Examples of luminescent pigments can be found in certain classes of inorganic compounds, such as the sulphides, oxysulphides, phosphates, vanadates, garnets, spinels, etc. of nonluminescent cations, doped with at least one luminescent cation chosen from the transition-metal or the rare-earth ions.
Another class of compound useful to produce luminescence in ink is formed by certain rare-earth metal complexes such as described in patent applications WO 2009/005733 or in U.S. Pat. No. 7,108,742.
A particular process for imprinting secure document with luminescent compounds, in particular luminescent rare-earth metal complexes, is inkjet printing, and more particularly thermal inkjet printing. Thermal inkjet printers use print cartridges having a series of tiny electrically heated chambers, constructed by photolithography. To produce an image, the printer sends a pulse of electric current through heating elements disposed in the back of each chamber, causing a steam explosion in the chamber, so as to form a bubble, which propels a droplet of ink through an orifice of the chamber onto the paper in front of it (hence the tradename Bubblejet® for certain inkjet printers). The ink's surface tension, as well as the condensation and thus contraction of the vapor bubble, pulls a further charge of ink into the chamber through a narrow channel attached to an ink reservoir.
The ink used is aqueous (i.e. a water-based ink comprising pigments or dyes), and the print head is generally cheaper to produce than the equipment required for other inkjet technologies. However, its lifetime is short, and it is generally exchanged together with the empty ink cartridge.
A major problem encountered with inkjet printers is ink drying in the printhead's nozzles, causing the pigments and/or dyes to form a solid deposit that plugs the microscopic ink orifices. Most printers prevent this drying by automatically covering the printhead nozzles with a rubber cap when the printer is not in use. Abrupt power loss, or unplugging the printer before it has capped its printhead, can, however, cause the printhead to dry out. Further, even when capped—this seal being not perfect—, over a period of several weeks, the ink in the nozzles can dry out and plug them. Once ink begins to dry out in the nozzles, the drop volume is affected, the drop trajectory can change, or the nozzle can completely fail to jet any ink.
In the case of luminescent inkjet inks comprising rare earth metal complexes, the stability of the complex in water is critical to avoid nozzle obstruction. In order to prevent premature drying, adding water or solvent, to sufficiently dilute the ink, is an obvious solution. However, dilution with water or solvent reduces the intensity of luminescence (and thus the ease of detection) of the security document printed with such ink.
Another problem encountered in thermal ink-jet printing is “Kogation”. Kogation (from Japanese “koge”=scorch, burn, char) is the thermal decomposition of ink components on the surface of the heating elements disposed in the back of each chamber of the ink-jet printing head, producing solid decomposition products, which may then obstruct the nozzle of the chamber.
Although rare earth metal complexes would represent a very useful way of imparting luminescence to inkjet inks, the problem of ink drying in the nozzles makes it often impossible to use the inkjet cartridges in their entirety, and causes thus increased ink cartridge consumption cost. This has not only an ecological and security impact, due to the “recycling” problem caused by such “used” cartridge, but also a non-negligible impact on the cost of printing.
Thus there is still a crucial need to solve the above mentioned problems in order to promote the efficient use of luminescent ink-jet inks based on rare earth metal complexes, and therefore to obtain correctly printed and protected security documents during the whole life of the ink cartridge.